The components of expense (income) for Pension Benefits and Other Retirement Benefits for the three and six months ended March 31, 2017 and 2016 are summarized as follows: | | | | | | | | | | | | | | | | | | Pension Benefits | | Other Retirement Benefits | | Three Months Ended | | Three Months Ended | | March 31 | | March 31 | (in millions) | 2017 | | 2016 | | 2017 | | 2016 | Service cost | $ | 3 |
| | $ | 3 |
| | $ | 1 |
| | $ | 1 |
| Interest cost | 27 |
| | 31 |
| | 2 |
| | 2 |
| Expected return on plan assets | (60 | ) | | (59 | ) | | (1 | ) | | (1 | ) | Amortization: | | | |
| | | | |
| Prior service credit | — |
| | (1 | ) | | — |
| | — |
| Net actuarial loss | 23 |
| | 20 |
| | 2 |
| | 2 |
| Net benefit expense (income) | $ | (7 | ) | | $ | (6 | ) | | $ | 4 |
| | $ | 4 |
|
| | | | | | | | | | | | | | | | | | Pension Benefits | | Other Retirement Benefits | | Six Months Ended | | Six Months Ended | | March 31 | | March 31 | (in millions) | 2017 | | 2016 | | 2017 | | 2016 | Service cost | $ | 6 |
| | $ | 6 |
| | $ | 1 |
| | $ | 1 |
| Interest cost | 55 |
| | 63 |
| | 3 |
| | 3 |
| Expected return on plan assets | (120 | ) | | (119 | ) | | (1 | ) | | (1 | ) | Amortization: | | | | | | | | Prior service credit | — |
| | (1 | ) | | — |
| | — |
| Net actuarial loss | 46 |
| | 39 |
| | 4 |
| | 4 |
| Net benefit expense (income) | $ | (13 | ) | | $ | (12 | ) | | $ | 7 |
| | $ | 7 |
|
|